1. Field of the Invention
The present invention relates to an active vibrational noise control apparatus for actively controlling vibrational noise with adaptive notch filters, and more particularly to an active vibrational noise control apparatus for use on vehicles.
2. Description of the Related Art
FIG. 15 of the accompanying drawings shows in block form an electric arrangement of a general active vibrational noise control apparatus 1 for actively controlling vibrational noise with an adaptive notch filter.
As shown in FIG. 15, the active vibrational noise control apparatus, generally denoted by 1, has an adaptive notch filter 2 and a reference signal generator 3 which are supplied with a base signal x(n) generated from the frequency of vibrational noise that is to be controlled.
The reference signal generator 3 generates and outputs a reference signal r(n) which takes into account transfer characteristics from a speaker 4 serving as a control sound source to a microphone 5 which outputs a residual noise signal e(n).
A filter coefficient updater 6 calculates and sequentially updates a filter coefficient W(n) of the adaptive notch filter 2 from the reference signal r(n) and the residual noise signal e(n) according to the equation [W(n+1)=W(n)+μe(n)·r(n): μ represents a constant] in order to minimize the residual noise signal e(n).
The adaptive notch filter 2 outputs a control signal y(n)=x(n)W(n) based on the filter coefficient W(n) and the base signal x(n).
In the active vibrational noise control apparatus 1, the base signal x(n), the filter coefficient W(n+1), the residual noise signal e(n), and the control signal y(n), etc. are generated or detected in each sampling period.
It is assumed that the fixed sampling technology with a fixed sampling period is employed, and the active vibrational noise control apparatus 1 has a control range (base signal frequency range) from 0 [Hz] to 1000 [Hz], for example, in which the base signal x(n) is generated with a resolution of 0.1 [Hz].
At a fixed sampling frequency of 4000 [Hz] (fixed sampling period of 0.25 [ms]), the active vibrational noise control apparatus 1 requires a data table (a storage means such as a memory) for storing discrete 40000 (=sampling frequency/resolution=4000/0.1) waveform data for generating the base signal x(n). Therefore, the active vibrational noise control apparatus 1 requires a storage means of large storage capacity and is costly to manufacture.
According to the conventional variable sampling technology with a sampling period being variable in synchronism with an engine rotational speed, if the number of discrete waveform data for generating the base signal x(n) is N, then in order to generate a base signal having a frequency in synchronism with the engine rotational speed, a sampling period ts (ts=Tnep/N) is calculated by dividing the period (base period Tnep) of engine pulses Pne in synchronism with the engine rotational speed by N, as shown in FIG. 16 of the accompanying drawings.
The base signal x(n) shown in a lower portion of FIG. 16 is generated depending on the sampling period ts.
According to the variable sampling technology, as the frequency of the base signal is lower, the number of noise canceling processes per second (=the number of updating processes or the number of calculations) is smaller. Consequently, the noise canceling capability varies in the control range. Since the number of discrete waveform data for generating the base signal x(n) is smaller than the number of discrete waveform data according to the fixed sampling technology, the storage means for storing the base signal may be of a smaller storage capacity. The number of discrete waveform data disclosed in Japanese Laid-Open Patent Publication No. 3-5255 is 180.
Noise control apparatus related to the variable sampling technology are disclosed in Japanese Laid-Open Patent Publication No. 3-5255 and Japanese Laid-Open Patent Publication No. 7-64575.
FIG. 17 of the accompanying drawings is a graph showing a control range according to the conventional variable sampling technology, the graph having a horizontal axis representative of the base period Tnep which is the base period of the base signal and a vertical axis representative of the sampling period ts. If the value produced by dividing the base period Tnep by the sampling period ts is referred to as a division number, then the division number is equal to the number of waveform data (N). Therefore, the sampling period ts can be determined as ts=Tnep/N from the base period Tnep along a sampling period curve C6 (C6=1/N) indicated by the thick solid line. Because as the base period Tnep is smaller, the sampling period ts is shorter, there is a trade-off problem between a sampling period tmin (=shortest sampling period=processing ability limit sampling period=lower limit sampling period) corresponding to the processing ability limit of a CPU of a microcomputer or the like and a base period Tnepmin (=base signal minimum period=base signal maximum frequency=maximum control frequency) at the lower limit of the control range.
In FIG. 17, tmax represents an upper limit sampling period (=longest sampling period=noise canceling ability limit sampling period) for achieving an effective noise canceling ability. If the sampling period ts is longer than the noise canceling ability limit sampling period tmax, then the number of noise canceling processes per second is so small that no desired noise canceling capability is available. In FIG. 17, Tnepmax represents an upper limit period (upper limit base period) of the base signal.
For performing effective noise control, it is necessary to equalize the minimum period of the base signal (lower limit base period) Tnepmin to the CPU processing ability limit sampling period (lower limit sampling period) and also to equalize the maximum period of the base signal (upper limit base period) Tnepmax to the noise canceling ability limit sampling period (upper limit sampling period) tmax. Therefore, if the control range is to be widened, then a fast high-performance CPU is needed, making the active vibrational noise control apparatus highly costly to manufacture.
The conventional variable sampling technology is also problematic in that since the number of waveform data and the division number are equal to each other, the number of waveform data and the division number N are a natural number, and the freedom with which to design the active vibration noise control apparatus is small.